Post-absorptive changes of the plasma amino acids reflect the adequacy of the balance of the dietary amino acids. Previous studies conducted in our laboratory utilizing man or dogs as test subjects have been successful in developing an in vivo plasma amino acid (PAA) ratio procedure to assess the ideal intake of dietary amino acids. Subsequent studies adapted this PAA ratio procedure to the rat in order to make it possible to determine the amino acid requirements for this species. By using the rat, it will be possible to determine the validity of these requirement data by carrying out long-term growth and balance studies. During the initial 21 months of this program, the amino acid requirements for the one, two, three, and six-month old rat have been determined utilizing the PAA ratio procedure. These data show a lower amino acid need for lysine and the sulfur amino acids for the one and two-month old rat than the published requirement data generally obtained by nitrogen balance and growth studies. Similar data utilizing the PAA ratio procedure will be obtained for the twelve-month old rat. Long-term growth and development studies have been initiated to determine whether these ideal dietary amino acid patterns as assessed by the PA ratio procedure are more exact in predicting the amino acids needs than the requirements as previously obtained by nitrogen balance procedures. Body growth and nitrogen retention of the rats fed a diet containing casein supplemented with amino acids to provide a dietary amino acid pattern based on the PAA ratio data were equal to or greater than the values obtained for rats fed casein supplemented with the amino acids to provide a dietary amino acid pattern based on requirement data obtained from nitrogen balance and growth studies. These long-term growth studies will be continued in the third year of this program to determine the validity of the PAA ratio procedure to estimate amino acid needs. BIBLIOGRAPHIC REFERENCES: Longenecker, John B., and Lo, Grace S., "Use of Plasma Amino Acid Ratio method to Assess Amino Acid Needs of Young Rats". Proceedings of the Xth International Congress of Nutrition, page 190, 1975.